This invention relates to a power factor monitoring and control system for resistance welding, and more particularly, to a control which senses the resistive drop that occurs in the formation of a quality weld in the secondary of a welding transformer by monitoring the corresponding change in the total load power factor as sensed in the primary of a welding transformer to provide an automatic heat control for improving weld quality over the tip life as well as reducing power consumption.
Resistance welding is now widely used in most applications involving the joining of metal, such as mild steels used in the manufacture of automobiles, and is recognized by all users as a most economic and rapid process when properly applied. Nevertheless, there are a number of parameters in the welding process that must be monitored in order to provide a quality weld. One such parameter which must be carefully monitored is the wear of the welding electrodes (tips) which must be compensated for by increasing the weld heat in order to insure good quality welds throughout the tip life.
Typically, prior art attempts at assuring weld quality despite tip wear (mushrooming) included the following monitoring (feedback) control techniques:
1. Optical (infrared), i.e., monitoring surface radiation to assess weld quality; PA1 2. Weld expansion, i.e., monitoring electrode displacement (thermo-expansion); PA1 3. Ultra-sonic, i.e., monitoring ultra-sonic transmissions transmitted through the weld area during the weld formation process; PA1 4. Weld energy, i.e., monitoring weld energy during the formation of the weld; PA1 5. Acoustic emmissions (expulsion detector) i.e., monitoring the acoustic emissions occurring during weld formation; and PA1 6. Resistance change i.e., monitoring the resistance change occurring during the weld formation.
Specifically, all of the above prior art techniques of monitoring weld quality were implemented by attaching or positioning various sensors and their respective leads in direct contact with the welding electrodes or in close proximity, thereto. Unfortunately, these monitoring devices and their leads attached to or in close proximity to the welding electrodes, that work so well in a laboratory environment when manned by expert technicians, seldom stood up in an industrial environment in which welding machines are sometimes manned by unskilled operators on an assembly line. The results in many cases were damaged monitoring devices as well as severed leads thereto which make it impossible to monitor the quality of the weld.
For the above stated reasons, monitoring devices and their leads attached to or in close proximity to the welding electrodes are often inadequate to assure good quality welds throughout the tip life because of continual maintenance problems. Examples of the above type of monitoring techniques and associated devices are contained in a publication entitled "Resistance Welding Control and Monitoring" published by The Welding Institute located at Abington Hall, Abington, Cambridge, CB 16Al, United Kingdom, copyrighted 1977.
One succesful way to compensate for electrode wear without attaching monitoring devices next to or on the welding tips is found in the digital welder control system of U.S. Pat. No. 4,104,724 ('724 patent). The controller of this patent provided a maintenance interval counter and compensator control having a 4-step, stepper. The stepper control of this patent is used to automatically increase the weld heat after a preset number of welds based on past experience to compensate for electrode mushrooming. Moreover, the digital welder control system of this patent is hereby incorporated by reference as to a type of digital welder control system that is ideally suited for modification to incorporate the features of the present invention.